Actually, we might be able to automatically spawn a safe fork server _only_ when people start mixing threading and subprocess.

I'm not totally sure if this would allow us to salvage multiprocessing as well...

The tricky bit is that we'd need to proxy into the fork server all the calls having to do with file descriptors / sockets that we would want to pass into the child processes.

That suggests to me that it'll be really hard to do this in a backwards-compatible way.

Given that subprocess is a pretty broken library, this might be a good time to replace it anyway.